1. Technical Field
The present invention relates to improvements in or relating to glaucoma management and treatment, and is more particularly concerned with glaucoma devices that integrate both intraocular pressure sensors and glaucoma drainage devices.
2. Description of the Related Art
The mammalian eye comprises an anterior chamber located between the cornea and the iris and lens. This chamber is filled with a fluid known as aqueous humour. A trabecular meshwork, comprising a plurality of microscopic passageways, is located in the angle between the iris and the cornea. In the normal human eye, aqueous humour is generated at a constant rate, typically about 2.7 microliters per minute (μl/min), by the ciliary body behind the iris. This aqueous humour flows past the lens and iris and then exits via the trabecular meshwork and is returned to the circulatory system.
The intraocular pressure (IOP) maintaining this flow in the normal eye tends to remain within a range of 10 mmHg to 20 mmHg. However, there may be significant changes in the IOP related to the cardiac cycle, blinking, diurnally and other causes. This makes it difficult to obtain representative values of the IOP from infrequent measurements. In the most prevalent chronic form of glaucoma where the iris-cornea angle remains open, there is blockage of the trabecular meshwork fluid outflow path which causes a build-up of excess fluid in the eye, and, consequently raises IOP to a value consistently greater than about 18 mmHg. In some cases, the IOP may be as high as 50 mmHg or more. Over time, this pressure increase results in irreversible damage to the optic nerve and loss of vision.
Glaucoma is a major cause of blindness worldwide and affects over 60 million people. Glaucoma is associated with many conditions including high blood pressure, diabetes, steroid use and ethnic origins. Various treatments are currently available for glaucoma including drug regimes, laser trabeculoplasty, trabeculotomy and trabeculectomy and intraocular drainage implants.
Drugs are frequently administered in the form of eye drops to control fluid inflow, that is, the formation of aqueous humour, or to open the trabecular meshwork. Erratic dosages, side effects and poor patient compliance are common issues.
In an alternative to drug use for glaucoma treatment, surgical creation of shunt paths or drains around or through the meshwork blockage is adopted as a means to release excess fluid and hence the build-up of IOP. In trabeculoplasty, a laser is used to create small openings in the trabecular meshwork of the eye so that aqueous humour can drain through the meshwork to reduce the intraocular pressure in the anterior chamber of the eye. This method of treatment is mainly used for open angle glaucoma.
Surgical techniques include trabeculotomy and trabeculectomy. Trabeculotomy is a surgical technique in which an opening is created in the trabecular meshwork using a small instrument to allow the fluid to flow from the anterior chamber. Trabeculectomy is the most common surgical technique for glaucoma in which part of the trabecular meshwork is removed. These methods allow fluid to collect under the conjunctiva and be reabsorbed by the eye.
Implanted devices are most often used where other treatment methods have become ineffective, but, more recently, have been proposed as first choice, alone or in conjunction with drug therapy. These implants comprise drainage devices that are inserted into the eye so that aqueous humour can be drawn through a drainage path and away from the anterior chamber. In the most commonly used implants, for example, Molteno implants and Baerveldt shunts, a drain path is formed by a tube placed between the anterior chamber and a fluid dispersion plate located usually supra-sclerally, below the conjunctiva. Dispersed fluid from the plate forms a pool or “bleb” that is gradually re-absorbed into the outer layers of the eye.
However, the drainage plates of such devices may often be large and rigid, causing fibrotic reactions with the surrounding tissue that can progressively reduce effectiveness. For example, plates may have areas as large as 425 mm2 and cover almost 25% of the surface area of the eye. Whilst dispersion plates can be curved to match the general shape of the external layers of the eye, these can only approximate to the actual shape of the eye of an individual patient. Mismatches in plate to sclera geometry can cause chronic microtrauma problems. Whilst more recent drain devices are of smaller size they are still rigid with the potential for trauma during insertion and in subsequent use.
By way of overcoming certain of these shortcomings, U.S. Pat. No. 6,102,045 describes a soft compact implant, with reduced fibrotic reaction, that lowers the intraocular pressure (IOP) in the eye. The implant, comprising a porous cellulosic membrane, extends into the anterior chamber of the eye, then through an opening in the limbus cornea to a drainage area underneath a scleral flap, and, preferably located between the sclera and choroid. Once implanted, fluid is absorbed from the drainage area into the choroidal vascular bed, enabling aqueous humour to be drained from the anterior chamber. Such soft devices, providing a drain or wick function, are often referred to as setons.
In detection and monitoring of glaucoma, routine ophthalmic examinations conducted on a yearly or other basis commonly include a measurement of IOP. However, this low frequency of measurement may fail to detect elevated pressure in a timely manner. Furthermore, a single measurement gives no indication of potentially damaging variations from day to day. The established “gold standard” tonometry instruments used require medical skill and are suited only for occasional use.
Several devices have been disclosed to more frequently measure IOP. In particular, certain of these use a pressure sensor surgically implanted within the eye, combined with external wireless, magnetic, optical or other means to interrogate the implant and obtain IOP readings.
It is most usual to deploy passive sensors, that is, sensors that do not need an internal power source, to avoid any battery chemicals in the eye. The interrogation methods then provide some externally applied form of energy to activate the implant when a reading is to be taken. The techniques often resemble those well understood in the radio frequency identification (RFID) field.
However, such sensors have the common limitation of requiring surgery specifically to implant them. In many cases, there may not be a medical risk-benefit justification for a surgical procedure. Therefore, IOP sensors combined with other ophthalmic devices, most commonly intraocular lenses (IOL), are utilised to avoid the need for additional surgery.
Many known IOP sensors are relatively large in the context of the eye dimensions and are necessarily rigid to obtain correct sensor and readout function. This risks optical path blockage and trauma. IOP sensors are described in U.S. Pat. No. 7,677,107, U.S. Pat. No. 6,939,299, U.S. Pat. No. 6,579,235, US-A-2009/0299216 and US-A-2009/0069648.
U.S. Pat. No. 7,677,107 describes a wireless pressure sensor that comprises an inductor-capacitor arrangement sandwiched between protective layers of impermeable polymeric materials. The portion of a first substrate is configured to form a cavity with a first capacitor plate, an inductor also being formed on the first substrate. A second substrate having a second capacitor plate formed thereon is attached to the first substrate and seals the cavity formed in the first substrate, the second substrate being movable with respect to the first substrate. Changes in pressure are detected by corresponding capacitance variations resulting in changes to the resonant frequency of the combination. The inductor element is necessarily held rigid to maintain a stable value and cannot be conformal to the individual patient anatomy.
U.S. Pat. No. 6,939,299 describes a micromachined chip on which an inductor-capacitor arrangement is mounted. The capacitor comprises an upper plate and a lower plate that are prepositioned during fabrication. Changes in fluid pressure in the eye are cause plate deflections and thus capacitance changes, detectable as a change of resonant frequency in a similar manner to U.S. Pat. No. 7,677,107.
U.S. Pat. No. 6,579,235 describes a passive intraocular pressure sensor that operates with a monitoring recorder that is worn by the patient in whose eye the sensor is implanted. Magnetic coupling provides a link between the sensor and the monitoring device.
US-A-2009/0299216 describes an intraocular pressure sensor that uses a capacitor and a reference chamber to carry out pressure comparisons within the eye. The reference chamber has a predetermined acceptable pressure that is compared with the sensed pressure to provide an output indicative of the pressure within the eye.
US-A-2009/0069648 describes a microlectromechanical system (MEMS) device that uses piezo-resistive elements for sensing changes in pressure.
An intraocular pressure sensor is also described in an article entitled “Wireless Intraocular Pressure Sensing Using Microfabricated Minimally Invasive Flexible-Coiled LC Sensor Implant” by Po-Jui Chen, Saloomeh Saati, Rohit Varma, Mark S. Humayun and Yu-Chong Tai, Journal of Microelectromechanical Systems, Vol. 19, No. 4 Aug. 2010. In this article, passive wireless sensing is described using an implanted wireless pressure sensor comprising an electrical LC circuit with resonant frequency varying under pressure. The sensor includes a bendable coil substrate that can be folded for implant and expanded once implanted, resuming a rigid shape. Inductive coupling is used for a wireless link between the implanted sensor and an external device.
Once glaucoma has progressed to a stage where a drainage device has become medically indicated, the need and benefit for continuous IOP monitoring is self apparent. It is therefore advantageous to combine the insertion of a glaucoma drainage device with a pressure sensor. This requires only one surgical procedure for inserting both devices and provides a glaucoma management system that can readily be monitored.
Such a combination is described in U.S. Pat. No. 7,678,065. In U.S. Pat. No. 7,678,065, an IOP sensor is implanted at the same time as a drainage device. However, the sensor is not integral with the drainage device and after insertion must be separately attached within the eye, making the complexity comparable to two separate procedures.
In an article entitled “Design of a Wireless Intraocular Pressure Monitoring System for a Glaucoma Drainage Implant” by Kakaday T, Plunkett M, Mclnness S, Li J S, Voelker N H, Craig J E, Proc ICEMB 23, pages 198 to 201, 2009, a IOP sensor implant is described that is attached to the external plate of a Molteno glaucoma drainage device. The IOP sensor consists of a MEMS capacitive pressure sensor and a planar inductor printed directly onto a flexible, biocompatible polyimide printed circuit board (PCB) to complete a parallel resonant circuit, the circuit frequency varying with pressure and being externally detectable. The sensor implant is encapsulated in a biomaterial, polydimethylsiloxane (PDMS—commonly referred to as silicone rubber), to protect against the aqueous environment. The IOP sensor relies on the plate holding a generally fixed shape with predictable deflection under the pressure of fluid entering the plate via the tube from the anterior chamber. Disadvantages with such an implant include the added thickness of the drainage plate which can acerbate blistering effects occurring from bleb formation, and the measurement is of pressure in the drain plate rather than direct measurement of the pressure in the anterior chamber.
Whilst IOP sensors and drainage devices are known as discussed above, these devices are separate and tend to be located in different portions of the eye because of their different functions.